Variable pitch propellers are employed in various applications including turboprop engines. Turboprop engines have proven to be desirable for aircraft because of high reliability and fuel efficiency. A turboprop engine uses a gas turbine engine to provide shaft power that rotates propeller blades. The blades provide thrust to propel an aircraft. The thrust is varied by changing the pitch of the blades. Forward thrust is achieved by rotating the blades to a positive angle. At cruise, the blades are adjusted to the intermediate positive position reducing engine torque and saving fuel. During landings, the blades are rotated to a negative angle to provide reverse thrust.
Propeller pitch for hydraulic applications is generally controlled through three devices: a propeller pitch control, a propeller governor, and a feathering valve. The propeller pitch control is operated through the use of a power lever connected to pitch control cam. The propeller pitch control allows the operator to adjust the blade pitch during ground operation, typically between the reverse and flight idle positions. The propeller governor automatically adjusts the blade pitch, typically between the flight idle and full power positions, to maintain a predetermined engine speed. The governor uses flyweights to mechanically start a chain reaction when engine speed increases above the predetermined speed by opening a pilot valve plunger to increase blade pitch. This increases engine load which will decrease engine speed back to the predetermined speed. Finally, the feathering valve is essentially a safety device to mechanically position the blade in the full feather position during emergencies. The device is a mechanically operated valve that, when opened, dumps the oil from the pitch control system and adjusts the blades to the full feather position.
Typically these systems operate a single acting propeller pitch control piston. In a single acting pitch system, the hydraulic fluid supplied by the prop governor or the pitch control is fed through a beta tube to bias the propeller pitch control piston against a spring. For a dual acting piston, two sources of oil are typically controlled by the same types of mechanisms mentioned above or other mechanical actuators connected to a spool type valve. These oil supplies are typically fed to the pitch control piston and react against each other to move the control piston.
While hydraulic pitch control systems are well known in the art and have good reliability, they require heavy and complicated actuation devices and associated linkages. Another problem is that the governor requires speed error to adjust the blade pitch resulting in fluctuations of blade pitch during transient conditions. Still another problem is that the pitch control cam requires large mechanical loads to operate.
Accordingly, a need exists for a pitch control device that can vary blade pitch from the reverse position to the feather position with a fewer number of components, that can control blade pitch during flight with minimum speed error, and that does not require great mechanical loads to operate.